Layout of circuit board

ABSTRACT

A layout of a circuit board includes a first signal layer, a second signal layer, and a third signal layer. The first signal layer has a transmission line. The second signal layer is stacked below the first signal layer, and has an opening. The third signal layer is stacked below the first and second signal layers with the second signal layer sandwiched between the first and third signal layers. The third signal layer is electrically connected to the second signal layer, and both of the second and third signal layers are ground layers or power layers. An orthogonal projection of a segment of the transmission line on the third signal layer is overlapped with that of the opening on the third signal layer. Therefore, an equivalent impedance of the segment of the transmission line referring to the second and third signal layers can be increased or decreased.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwan application serial no. 96135143, filed on Sep. 20, 2007. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a layout of a circuit board, in particular, to a layout of a circuit board capable of changing impedance values at both ends of one transmission line.

2. Description of Related Art

Circuit boards have been widely applied in various electronic devices. Many different electronic components are mounted on a circuit board, and are electrically connected to each other through the layout of the circuit board. Moreover, the heat generated by the electronic components is partially dissipated by metal portions of the circuit boards. In actual application, the mainboard of a notebook computer is an application of the circuit board with various electronic components mounted thereon.

In order to adjust an equivalent impedance at both ends of a segment of a transmission line in the circuit board referring to a ground layer or power layer located beside the transmission line to a particular value. Conventionally, the line thickness, line width, or line pitch of the segment of the transmission line, or the distance between the segment of the transmission line and the adjacent reference plane thereof may be changed to achieve the purpose of adjusting the equivalent impedance at both ends of the segment of the transmission line.

However, in order to increase the equivalent impedance of the segment of the transmission line referring to a ground layer or power layer, the line thickness, line width, or line pitch of the segment of the transmission line in the circuit board cannot be reduced under the limitation of the precision and yield of the process. Moreover, restricted by the thinning of the circuit board, the distance between the segment of the transmission line and the adjacent reference plane thereof cannot be increased. Therefore, the current layout of a circuit board is limited on design.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a layout of a circuit board, which is capable of increasing an equivalent impedance of a segment of a transmission line referring to a ground layer or power layer.

The present invention provides a layout of a circuit board, which includes a first signal layer, a second signal layer, and a third signal layer. The first signal layer has a transmission line. The second signal layer is stacked below the first signal layer, and has an opening. The third signal layer is stacked below the first and second signal layers with the second signal layer sandwiched between the first and third signal layers. The third signal layer is electrically connected to the second signal layer, and both of the second and third signal layers are ground layers or power layers. An orthogonal projection of a segment of the transmission line on the third signal layer is overlapped with that of the opening on the third signal layer.

In an embodiment of the present invention, a width of the orthogonal projection of the segment of the transmission line on the third signal layer is smaller than that of the orthogonal projection of the opening on the third signal layer.

In an embodiment of the present invention, the first signal layer further includes another transmission line, forming a differential pair with the above transmission line. An orthogonal projection of a section of the differential pair on the third signal layer is overlapped with that of the opening on the third signal layer. Moreover, a width of the orthogonal projection of the section of the differential pair on the third signal layer is smaller than that of the orthogonal projection of the opening on the third signal layer.

In an embodiment of the present invention, an equivalent impedance of the segment of the transmission line is greater than that of any other segment of the transmission line.

The present invention further provides a layout of a circuit board, which includes a first signal layer, a second signal layer, a third signal layer, and a ground line. The first signal layer has a transmission line. The second signal layer is stacked below the first signal layer, and has an opening. The third signal layer is stacked below the first and second signal layers with the second signal layer sandwiched between the first and third signal layers. The ground line is set on the same level as the third signal layer, and electrically connected to the second signal layer. In addition, an orthogonal projection of a segment of the transmission line on the ground line is overlapped with that of the opening on the ground line.

In an embodiment of the present invention, a width of the orthogonal projection of the segment of the transmission line on the ground line is smaller than that of the orthogonal projection of the opening on the ground line.

In an embodiment of the present invention, an equivalent impedance of the segment of the transmission line is greater than that of any other segment of the transmission line. In the present invention, an opening is formed in the second signal layer to increase the distance between the segment of the transmission line and the third signal layer (i.e., a ground layer or power layer), so as to increase an equivalent impedance of the segment of the transmission line referring to the second and third signal layers, thereby meeting the requirement on the design of the circuit board. Further, in the present invention, an opening is formed in the second signal layer and a ground line is set on the same level as the third signal layer, so as to increase an equivalent impedance of the segment of the transmission line referring to the second signal layer and the ground line, so as to meet the requirement on the design of the circuit board. In addition, the present invention may also be applied to a circuit board having a differential pair.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

FIG. 1A illustrates a layout of a circuit board according to an embodiment of the present invention.

FIG. 1B is an exploded view of the layout in FIG. 1A.

FIG. 1C is a top view of the layout in FIG. 1B.

FIG. 1D is a cross-sectional view of the circuit board in FIG. 1A.

FIG. 1E is a cross-sectional view of a layout of a circuit board according to still another embodiment of the present invention.

FIG. 2A illustrates a layout of a circuit board according to another embodiment of the present invention.

FIG. 2B is an exploded view of the layout in FIG. 2A.

FIG. 2C is a top view of the layout in FIG. 2B.

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the present embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.

In the following two embodiments, the same symbols represent identical or similar components.

FIG. 1A illustrates a layout of a circuit board according to an embodiment of the present invention, FIG. 1B is an exploded view of the layout in FIG. 1A, and FIG. 1C is a top view of the layout in FIG. 1B. Referring to FIGS. 1A to 1C, the circuit board 10 of this embodiment has a layout 100. The layout 100 includes a first signal layer 110, a second signal layer 120, and a third signal layer 130 stacked in sequence with the second signal layer 120 sandwiched between the first signal layer 110 and the third signal layer 130. Further, the circuit board 10 includes a plurality of dielectric layers 20 respectively located between the signal layers 110, 120, 130, for electrically insulating the signal layers 110, 120, 130, as shown in FIG. 1A.

The first signal layer 110 has a transmission line 112, and the second signal layer 120 is a ground layer. In order to increase an equivalent impedance of a segment 112 a of the transmission line 112 with respect to the second signal layer 120, an opening 122 is formed in the second signal layer 120, and the third signal layer 130 is electrically connected to the second signal layer 120. In this embodiment, the third signal layer 130 is electrically connected to the second signal layer 120 through a plurality of conductive vias 140 (only one conductive via 140 is shown in FIG. 1B).

The third signal layer 130 and the second signal layer 120 are ground layers or power layers to be referred by the segment 112 a of the transmission line 112.

An orthogonal projection of the segment 112 a of the transmission line 112 on the third signal layer 130 is overlapped with that of the opening 122 on the third signal layer 130. In other words, an orthogonal projection of the segment 112 a of the transmission line 112 on the second signal layer 120 is overlapped with the opening 122. In this embodiment, a width W1 of the orthogonal projection of the segment 112 a of the transmission line 112 on the third signal layer 130 is smaller than a width W2 of the orthogonal projection of the opening 122 on the third signal layer 130.

The opening 122 is formed in the second signal layer 120 to increase the distance between the segment 112 a of the transmission line 112 and the third signal layer 130, such that an equivalent impedance of the segment 112 a of the transmission line 112 referring to the second signal layer 120 and the third signal layer 130 is greater than that of any other segment of the transmission line 112 referring to the second signal layer 120. Therefore, an equivalent impedance of any segment obtained from the transmission line 112 may be adjusted, so as to meet the requirement on the design of the circuit board 10.

FIG. 1D is a cross-sectional view of the circuit board in FIG. 1A. Referring to FIGS. 1C and 1D, the opening 122 is formed to increase the distance between the segment 112 a of the transmission line 112 and a ground layer (power layer), and the segment 112 a of the transmission line 112 takes the third signal layer 130 (also a ground layer) as reference. In this embodiment, the equivalent impedance is obtained by the following formula:

${Zo} = {\frac{87}{\sqrt{{Er} + 1.41}}\ln \frac{5.98h}{{0.87w} + t}}$

In the above formula, Zo is the equivalent impedance, Er is the relative dielectric constant, h is the distance between a transmission line and a ground layer, w is the width of the transmission line, and t is the thickness of the transmission line. The dielectric layer 20 is made of an epoxy glass with a relative dielectric constant of 4.8 to 5.0. When the width w and thickness t are fixed, the distance h is in direct proportion to the equivalent impedance Zo.

When the opening 122 is not formed, the segment 112 a of the transmission line 112 takes the second signal layer 120 (a ground layer/power layer) as reference. The distance between the segment 112 a of the transmission line 112 and the second signal layer 120 is h1. However, when the opening 122 is formed, the segment 112 a of the transmission line 112 takes the third signal layer 130 (a ground layer/power layer) as reference. The distance between the segment 112 a of the transmission line 112 and the third signal layer 130 is h2.

Thus, through the opening 122 in this embodiment, the distance h2 between the segment 112 a of the transmission line 112 and the third signal layer 130 is greater than the distance h1 between any other segment of the transmission line 112 and the second signal layer 120, such that the equivalent impedance of the segment 112 a of the transmission line 112 is greater than that of any other segment of the transmission line 112.

FIG. 1E is a cross-sectional view of a layout of a circuit board according to another embodiment of the present invention. Referring to FIG. 1E, compared with the layout 100 in FIG. 1D, the layout 100′ of a circuit board of this embodiment further includes a ground line 122 a. The ground line 122 a is set on the same level as the third signal layer 130, and is electrically connected to the second signal layer 120 through at least one conductive via 140. An orthogonal projection of a segment 112 a of the first transmission line 112 in the signal layer 110 on the ground line 122 a is overlapped with that of the opening 122 in the second signal layer 120 on the ground line 122 a. Likewise, the equivalent impedance of the segment 112 a of the transmission line 112 is greater than that of any other segment of the transmission line 112. In this embodiment, a width of the orthogonal projection of the segment 112 a of the transmission line 112 on the ground line 122 a is smaller than that of the orthogonal projection of the opening 122 on the ground line 122 a, and the third signal layer 130 is a signal layer for signal transmission.

FIG. 2A illustrates a layout of a circuit board according to another embodiment of the present invention, FIG. 2B is an exploded view of the layout in FIG. 2A, and FIG. 2C is a top view of the layout in FIG. 2B. Referring to FIGS. 2A to 2C, the circuit board 10A of this embodiment has a layout 100A. The layout 100A includes a first signal layer 110A, a second signal layer 120, and a third signal layer 130 stacked in sequence with the second signal layer 120 sandwiched between the first signal layer 110A and the third signal layer 130. Moreover, the circuit board 10A further includes a plurality of dielectric layers 20 respectively sandwiched between the signal layers 110A, 120, 130, for electrically insulating the signal layers 110A, 120, 130, as shown in FIG. 2A.

In this embodiment, the first signal layer 110A has transmission lines 112, 114 forming a differential pair 116. In addition, the second signal layer 120 is a ground layer or power layer. In order to increase a total equivalent impedance of a section 116 a of the differential pair 116 with respect to the second signal layer 120, an opening 122 is formed in the second signal layer 120, and the third signal layer 130 is electrically connected to the second signal layer 120. In this embodiment, the third signal layer 130 is electrically connected to the second signal layer 120 through a plurality of conductive vias 140 (only one conductive via 140 is shown in FIG. 2B).

In this embodiment, the third signal layer 130 and the second signal layer 120 are ground layers or power layers to be referred by the section 116 a of the differential pair 116.

An orthogonal projection of the section 116 a of the differential pair 116 on the third signal layer 130 is overlapped with that of the opening 122 on the third signal layer 130. In other words, an orthogonal projection of the section 116 a of the differential pair 116 on the second signal layer 120 is overlapped with the opening 122. In this embodiment, a width W3 of the orthogonal projection of the section 116 a of the differential pair 116 on the third signal layer 130 is smaller than a width W2 of the orthogonal projection of the opening 122 on the third signal layer 130.

The opening 122 is formed in the second signal layer 120 to increase the distance between the section 116 a of the differential pair 116 and the third signal layer 130, such that the equivalent impedance of the section 116 a of the differential pair 116 referring to the second signal layer 120 and the third signal layer 130 is greater than that of any other section of the differential pair 116 referring to the second signal layer 120. Therefore, the equivalent impedance of any section of the differential pair 116 formed by the transmission lines 112, 114 may be adjusted, so as to meet the requirement on the design of the circuit board 10A.

In view of the above, according to the present invention, an opening is formed in the second signal layer (a ground layer/power layer) at a position right below the segment of the transmission line in the first signal layer, for increasing the distance between the segment of the transmission line and the ground layer (power layer), such that the equivalent impedance of the segment of the transmission line referring to the second signal layer and the third signal layer (a ground layer/power layer) which is electrically connected to the second signal layer and located below the second signal layer is increased. Further, in the present invention, an opening is formed in the second signal layer (a ground layer/power layer) at a position right below the segment of the transmission line in the first signal layer, and a ground line is disposed below the second signal layer and electrically connected to the second signal layer, for increasing the distance between the segment of the transmission line and the ground layer (power layer), such that the equivalent impedance of the segment of the transmission line referring to the second signal layer and the ground line located below the second signal layer is increased.

In addition, the present invention is also applicable to a circuit board having a differential pair, so as to increase the equivalent impedance of a certain section of the differential pair referring to a ground layer/power layer.

It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents. 

1. A layout of a circuit board, comprising: a first signal layer, comprising a transmission line; a second signal layer, stacked below the first signal layer, and comprising an opening; and a third signal layer, stacked below the first and second signal layers with the second signal layer sandwiched between the first and third signal layers, wherein the third signal layer is electrically connected to the second signal layer, and both of the second and third signal layers are ground layers, and an orthogonal projection of a segment of the transmission line on the third signal layer is overlapped with that of the opening on the third signal layer.
 2. The layout of a circuit board according to claim 1, wherein a width of the orthogonal projection of the segment of the transmission line on the third signal layer is smaller than that of the orthogonal projection of the opening on the third signal layer.
 3. The layout of a circuit board according to claim 1, wherein the first signal layer further comprises another transmission line, forming a differential pair with the transmission line, and an orthogonal projection of a section of the differential pair on the third signal layer is overlapped with that of the opening on the third signal layer.
 4. The layout of a circuit board according to claim 3, wherein a width of the orthogonal projection of the section of the differential pair on the third signal layer is smaller than that of the orthogonal projection of the opening on the third signal layer.
 5. The layout of a circuit board according to claim 1, wherein an equivalent impedance of the segment of the transmission line is greater than that of any other segment of the transmission line.
 6. A layout of a circuit board, comprising: a first signal layer, comprising a transmission line; a second signal layer, stacked below the first signal layer, and comprising an opening; a third signal layer, stacked below the first and second signal layers with the second signal layer sandwiched between the first and third signal layers; and a ground line, set on the same level as the third signal layer, and being electrically connected to the second signal layer, wherein an orthogonal projection of a segment of the transmission line on the ground line is overlapped with that of the opening on the ground line.
 7. The layout of a circuit board according to claim 6, wherein a width of the orthogonal projection of the segment of the transmission line on the ground line is smaller than that of the orthogonal projection of the opening on the ground line.
 8. The layout of a circuit board according to claim 6, wherein an equivalent impedance of the segment of the transmission line is greater than that of any other segment of the transmission line.
 9. The layout of a circuit board according to claim 6, wherein the ground line is connected to the second signal layer through a conductive via.
 10. The layout of a circuit board according to claim 6, wherein the second signal layer is a ground layer.
 11. The layout of a circuit board according to claim 6, wherein the second signal layer is a power layer.
 12. A layout of a circuit board, comprising: a first signal layer, comprising a transmission line; a second signal layer, stacked below the first signal layer, and comprising an opening; and a third signal layer, stacked below the first and second signal layers with the second signal layer sandwiched between the first and third signal layers, wherein the third signal layer is electrically connected to the second signal layer, an both of the second and third signal layers are power layers, and an orthogonal projection of a segment of the transmission line on the third signal layer is overlapped with that of the opening on the third signal layer.
 13. The layout of a circuit board according to claim 12, wherein a width of the orthogonal projection of the segment of the transmission line on the third signal layer is smaller than that of the orthogonal projection of the opening on the third signal layer.
 14. The layout of a circuit board according to claim 12, wherein the first signal layer further comprises another transmission line, forming a differential pair with the transmission line, and an orthogonal projection of a section of the differential pair on the third signal layer is overlapped with that of the opening on the third signal layer.
 15. The layout of a circuit board according to claim 14, wherein a width of the orthogonal projection of the section of the differential pair on the third signal layer is smaller than that of the orthogonal projection of the opening on the third signal layer.
 16. The layout of a circuit board according to claim 12, wherein an equivalent impedance of the segment of the transmission line is greater than that of any other segment of the transmission line. 